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Ubuntu Security Notice USN-6168-2
June 20, 2023

libx11 vulnerability
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A security issue affects these releases of Ubuntu and its derivatives:

- Ubuntu 18.04 LTS (Available with Ubuntu Pro)
- Ubuntu 16.04 LTS (Available with Ubuntu Pro)
- Ubuntu 14.04 LTS (Available with Ubuntu Pro)

Summary:

libx11 could be made to crash if it received specially crafted network
traffic.

Software Description:
- libx11: X11 client-side library

Details:

USN-6168-1 fixed a vulnerability in libx11. This update provides
the corresponding update for Ubuntu 14.04 ESM, Ubuntu 16.04 ESM,
and Ubuntu 18.04 ESM.

Original advisory details:

 Gregory James Duck discovered that libx11 incorrectly handled certain
 Request, Event, or Error IDs. If a user were tricked into connecting to a
 malicious X Server, a remote attacker could possibly use this issue to
 cause libx11 to crash, resulting in a denial of service.

Update instructions:

The problem can be corrected by updating your system to the following
package versions:

Ubuntu 18.04 LTS (Available with Ubuntu Pro):
  libx11-6                        2:1.6.4-3ubuntu0.4+esm1

Ubuntu 16.04 LTS (Available with Ubuntu Pro):
  libx11-6                        2:1.6.3-1ubuntu2.2+esm2

Ubuntu 14.04 LTS (Available with Ubuntu Pro):
  libx11-6                        2:1.6.2-1ubuntu2.1+esm3

After a standard system update you need to reboot your computer to make all
the necessary changes.

References:
  https://ubuntu.com/security/notices/USN-6168-2
  https://ubuntu.com/security/notices/USN-6168-1
  CVE-2023-3138